In the fundamental structure of a solid electrolytic capacitor, a high-density and uniform oxide dielectric film is formed on the surface of a valve-acting metal previously etched to roughen the surface, such as aluminum, tantalum, titanium or niobium, an electrically conducting polymer, for example, is formed on the oxide dielectric film to work out to a solid electrolyte, an anode lead wire is connected to the anode terminal (the metal surface area with no solid electrolyte) of the valve-acting metal, and a cathode lead wire is connected to the electrically conducting layer comprising the electrically conducting polymer. This structure as a whole is then sealed with an insulating resin such as epoxy resin to complete a solid electrolytic capacitor.
Among the above-described valve-acting metals, aluminum is advantageous in that the surface area can be easily enlarged by etching, the oxide film formed on its surface by anodization (electrochemical forming) using the aluminum as anode can be utilized as a dielectric material and therefore, a smaller-size and larger-capacitance solid electrolytic capacitor can be less expensively produced as compared with other capacitors. Therefore, the aluminum solid electrolytic capacitor is being widely used.
The etching of aluminum is generally performed by A.C. (alternating current) etching in an electrolytic solution containing chlorine ion and the like. By this etching, a large number of pores are formed on the surface and the surface area is enlarged. The radius of the pore thus formed varies depending on the current applied and the etching time, however, it is approximately from 0.05 to 1.0 μm.
The surface including the pores is then subjected to anodization (electrochemical forming). By this electrochemical forming, a high-density and uniform anode oxide film (dielectric film) having a thickness of approximately from 0.005 to 0.1 μm is formed.
The formed aluminum substrate obtained is cut into the predetermined size of a solid electrolytic capacitor. At this time, an extruded portion (bur) remains at the cut end edge, however, this exposed aluminum (ground metal) portion is again electrochemically formed as it is to form an anode oxide film (dielectric film) on the cut end part.
As for the method of increasing an electrostatic capacitance, JP-B-57-6250 (the term “JP-B” as used herein means an “examined Japanese patent publication”) describes a technique of subjecting a formed or etched foil to a boiling treatment (hot water treatment) with an aqueous sodium silicate solution. This method is effective for a formed foil obtained by the electrochemical forming at a forming voltage of 20 to 300 V but fails in increasing the electrostatic capacitance of a low-voltage foil obtained at a forming voltage of less than 20 V.
Also, Capacitor Gijutsu (Capacitor Techniques), Vol. 8 (No. 1), pp. 21–28, Denki Kagaku Kai (2001) (First Research Meeting in 2001) describes a technique of a sol-gel coating or the like for the development of aluminum electrolytic capacitors. According to this method, SiO2 or the like is sol-gel covered and anodized in a neutral solution to form a composite oxide of Al and Si. The composite oxide of Al and Si is formed between SiO2 layer and Al2O3 layer at 200 V, and the SiO2 layer disappears at 400 V. As a result, an electrolytic capacitor improved in the dielectric constant and increased in the capacitance is produced.
The electrostatic capacitance of a capacitor device is determined by the thickness of the dielectric film, the dielectric constant of the dielectric film and the area coverage of a solid electrolyte (electrically conducting substance) on the dielectric film. However, the electrostatic capacitance of conventional aluminum solid electrolytic capacitors does not agree with the theoretical electrostatic capacitance (C) of a formed aluminum foil (C=εA/t, wherein ε is a dielectric constant of an aluminum oxide dielectric material, A is a surface area of a dielectric layer and t is a thickness of a dielectric material). Moreover, the electrostatic capacitance is greatly dispersed among individual products.
As the electrochemical forming voltage decreases, the electrostatic capacitance of an aluminum solid electrolytic capacitor using the electrochemically formed foil is liable to estrange at a larger ratio from the theoretical electrostatic capacitance of the formed aluminum foil. This phenomenon is considered to occur because the thickness and dielectric constant of the dielectric film, the area coverage and adhesion of the solid electrolyte (electrically conducting substance) on or to the dielectric film, and the like are insufficient in conventional electrochemical forming techniques.